Geology and geochronology of the Mesoproterozoic ca. 1.5 Ga McDowell Mountains Group, central Arizona, USA

The 6-km-thick, ca. 1585−1550 Ma McDowell Mountains Group of central Arizona, USA, may be one of the thickest and best-preserved sequences of mostly volcanic rocks of this age yet identified on Earth. It is comprised of three ash-flow tuffs—the Thompson Peak tuff, the 1572.7 ± 4.8 Ma Medio tuff, and the 1550.9 ± 3.7 Ma Taliesin tuff, intercalated with volumetrically minor 1568 ± 5.0 Ma rhyolite and autoclastic breccia, quartz-rich sediment exhibiting a maximum depositional age of 1558.3 ± 7.3 Ma, and metabasalt. These new ages, in addition to detrital zircon analyses, place better constraints on the deposition of the underlying Mazatzal Group to between ca. 1635 Ma and 1595 Ma. Both the McDowell Mountains Group and the Mazatzal Group underwent northwest-vergent thrust faulting, followed by intrusion of felsic and mafic sills, all of which were then buried, folded, intruded by granite plutons, and foliated under greenschist-facies metamorphic conditions. Intrusion of the Carefree granite at ca. 1422 Ma truncated almost all of the tectonic and metamorphic fabrics in the metamorphic rocks, indicating that thrusting and deformation occurred sometime prior to ca. 1422 Ma. The Carefree granite is cut by an extensive, yet very narrow (1−3-m-thick) mylonitic shear zone. Titanite and apatite U-Pb intercept ages in this shear zone, and within other rocks, point to a subsequent deformation event sometime between 1407 Ma and 1382 Ma and again between 1283 Ma and 911 Ma. Rare earth elements indicate that the magmas formed at an active continental margin. The initial εHf values of felsic volcanic rocks in the McDowell Mountains Group range from +1.9 to +7.4 and are very similar to the εHf values of detrital zircons from the Belt Supergroup and the Yankee Joe Group, suggesting that the McDowell Mountains Group may have been a Laurentian source for these deposits.